I'm going old school for this version of the made-up team, back to the original NES. It was a little tricky, between my not having actually owned all that many NES games, and several of the ones I did own were licensed properties (read: Teenage Mutant Ninja Turtle games, mostly), so a somewhat limited field of candidates.
The Leader: The Paperboy (Paperboy) - He probably would have fit well as The Rogue, but I didn't feel like I had a better option. And what we know about this kid is he's dedicated and will see a job through, not to mention fearless to the point of being crazy. He gets repeatedly attacked by the personification of Death and throws newspapers at it, like it was some stupid yappy dog. He's good at snap decisions and quick reactions.
Now, will all that translate into being able to lead a team? It's questionable, and if this team were comprised of more abrasive personalities, it could be a problem. Especially if a mission or quest blows up in their faces because the remarkable paperboy didn't have a clue what to do, or decided to be petty or just plain mean at the wrong time. Because he could be extremely dedicated and responsible, but he could also decide he was going to throw papers through every window on the street, and at everyone he saw along the way.
Little Mac, little guy, big punch, pretty solid head all things being equal. Yeah, you can one-shot KO him if you catch him with the a good uppercut, but consider that those are usually being delivered by people two or even three times heavier than him. If he fought people in his own weight class, he'd probably be undefeated.
Not that he can count on weight class restrictions on this team, or any kinds of Marquis of Queensbury rules. Hes gonna have to slug it out with people with weapons, armor, and no compunction against continuing to punch him once hes down. On the other hand, there's nothing stopping him from doing that either, and nothing constraining him to a ring either. Not that Mac seems fond of moving much. A little to his left or right, jump to throw a knockout shot, that's about it. He's going to have to show a little more flexibility, get outside the box, er, ring. I mean, what was all that running in the pink sweatsuit for if he's going to stay rooted in one place?
At least we know he has no problems taking orders from a guy on a bicycle.
One of his roles isn't so much a game of chance as him simply presenting Mario with three treasure chests and telling him to pick one for some sort of power-up. It would really be more efficient in Mario's quest to defeat Bowser if Toad just told him what was in the chests, or hell, gave him all three, but Toad's no doubt looting the Princess' personal armory/treasury for this stuff, and doesn't want to be any more wasteful than he has to be.
So it's hard telling what Toad will specifically bring to the battle. Will the Paperboy get himself a racoon tail and the ability to fly? Can Little Mac get a mushroom and turn giant to lay the smack down? Or will Toad pull out that stupid Music Box when there are no Hammer Brothers around to put to sleep with it? Can anyone on this team convince him to cut the bullshit and just give them the best option, or is it Toad doesn't even know what that is? He takes orders, he doesn't make decisions. Maybe he can't, or won't.
Kirby landed this role because of the Warp Star. Despite its tendency to drop off passengers by crashing into the Earth and send them tumbling, it still seems easier to control and direct than a Warp Whistle. And the kid's bike can only hold so many. Kirby is a wild card on his own, given his ability to inhale enemies and swipe their powers. Turn into a fireball, shoot lasers, gain a hammer, an umbrella, or even a UFO (that UFO power was sweet). Of course he could get a dud, but Kirby usually learns quickly which powers come from which enemies. And even if he finds an enemy he can't take powers from, he could still digest them, or spit them out at another foe. Plus he looks adorable, so it's a good bet enemies will underestimate him initially.
As for their enemy, it's a treacherous one. It's going to stir up trouble in one place initially, but in such a way the team will have to either split up to handle it, or prioritize one problem over the other. Threats to a city water supply that requires to different, hard-to-acquire compounds to neutralize. Igniting wars by stealing artifacts, while also undermining all the involved parties' leaders. And what's worst is the enemy will have presented itself as a friend an ally. When our team succeeds in thwarting his plan, he'll openly share in their successes. And when they fail, he's going to be laughing at them.